PD-1/PDL-1 signaling pathway is essential in the regulation of immune tolerance, microbial infection and tumor immune evasion. PD-1 (programmed cell death 1) is mainly expressed on T cells and other immune cells, and its ligand PDL-1 is highly expressed in many human tumor types. The presence of PDL-1 protein has been demonstrated by immunohistochemical analysis in human breast cancer, lung cancer, gastric cancer, colorectal cancer, esophageal cancer, ovarian cancer, cervical cancer, renal cell carcinoma, bladder cancer, pancreatic cancer, glioma and melanoma. Moreover, the expression level of PDL-1 is closely related to the clinical treatment and prognosis of a patient.
Blocking PD-1/PDL-1 signaling pathway can activate inhibited T cells, and induce activated T cells to attack cancer cells. Blocking PD-1/PDL-1 signaling can promote the proliferation of tumor antigen specific T cells which play a role in killing tumor cells, and then inhibit the growth of local tumor (Julie R et al., 2012, N Engl J Med. 366: 2455-2465); PDL-1 monoclonal antibody can up regulate the secretion of IFN-γ by tumor infiltrating CD8+ T cells, indicating that the blockade of the PD-1/PDL-1 signaling pathway plays a role in the immune response of tumor cells in order to induce the immune response (Blank C et al., 2006, Int. J. Cancer. 119:317-327).
In addition, PDL-1 can also bind to B7-1 in vivo. Studies have shown that the PDL-1/B7-1 complex is a negative signal for T cell activation, and the interaction can lead to the down-regulation of T cell surface activation markers, and inhibit the proliferation of T cells.
IL-2 (Interleukin-2) is a kind of lymphokine secreted by Th cells, and has a wide range of immune activities: {circle around (1)} stimulating the proliferation and differentiation of T cells; {circle around (2)} stimulating the generation of cytotoxic T lymphocytes; {circle around (3)} stimulating the proliferation and differentiation of NK cells and enhance the activity of NK cells; {circle around (4)} stimulating the generation of lymphokine activated killer cells (LAK cells) which is a type of tumor killing immune cells transformed from lymphocytes under the stimulation of IL-2 for 3-6 days in vitro. IFN-γ (Interferon-gamma) is produced by T cells, and it can inhibit the proliferation of tumor cells, increase the presentation of antigen by MHC, stimulate the expression of tumor necrosis factor, and prevent tumor angiogenesis. Recent studies reported that IFN-γ can suppress the ability of tumor cells to evade attacks from immune system by regulating the expression of Fas/FasL of tumor cells and enhancing the sensitivity of tumor cells to Fas mediated apoptosis, leading to the inhibition of the malignant tumor cells.
Currently, it is generally believed that antibodies targeting the PDL-1 pathway will lead to breakthrough in the treatment of a variety of tumors, including non-small cell lung cancer, renal cell carcinoma, ovarian cancer, melanoma (Homet M. B., Parisi G., et al., Anti-PD1 Therapy in Melanoma. Semin Oncol. 2015 June; 42(3):466-473), leukemia and anemia (Held S A, Heine A, et al., Advances in immunotherapy of chronic myeloid leukemia CML. Curr Cancer Drug Targets. 2013 September; 13(7):768-74).
At present, it is still necessary to develop a new anti-PDL-1 antibody with better binding affinity and blocking efficiency (PDL-1 to PD-1) to activate T lymphocytes.